Following a march by 12,000 people in the prosperous port of Dalian on Sunday, the city’s top official, Tang Jun, promised to shut down the Fujia plant that makes the chemical paraxylene, or PX, before moving it out of the downtown area.

The protest was mounted after waves from Tropical Storm Muifa last week broke a dike guarding the plant and raised fears that floodwaters could release toxic chemicals.

While no firm date for the move was given, the concession marked a rare about-face for the authoritarian government and underscored the power of the rising middle class, who enjoy relative wealth and high education levels and are fond of communicating and organizing using social media and text messaging.

“The Dalian government has learned a lesson from the previous mass incidents and may think it will help make the incident subside by dealing with the matter quickly,” Liu Shanying, a researcher at the Institute of Political Science of the Chinese Academy of Social Sciences, said Monday.

The move comes after a summer of discontent in China, with ethnic violence in Xinjian in the northwest, protests over authorities’ rough treatment of citizens in the southern city of Guizhou, and widespread criticism of the government’s handling of a high-speed rail crash that killed 40.

On Monday, more than 100 police officers including about a dozen riot police patrolled the public square in front of government offices in Dalian, a center for high-tech industries routinely rated one of China’s most livable cities.

The local Chinese-language media, which is state controlled, made no mention of the protests, and messages about it online were removed.

There was no sign that people were gathering to protest for a second day and only light security outside the factory, which police said was in the process of shutting down.

Sunday’s protest had led to some scuffles between police and demonstrators, but was mostly peaceful, in sharp contrast with other recent outbreaks of unrest, where police have been attacked and marchers arrested.

That was a reflection both of the middle class makeup of the protesters and the relatively calm, unthreatening, and apolitical nature of the protest. Elsewhere in China, public anger has led to violent demonstrations in recent weeks over corruption and abuse of power by local officials, drawing swift police crackdowns.

The Dalian protest drew participants from a broad sweep of society and focused on an issue of common concern for all ages and professional backgrounds, including rank and file party members and government bureaucrats, said Yang Yang, a political scientist at China University of Political Science and Law.

“Dalian has a lot of wealthy, upper class people and their influence over the government is far greater than the ordinary people,” Yang said. “It’s no surprise that the project was canceled amid the public anger.”

As with all such incidents, speculation churned about the role of politics in the protest.

Fu Jia and its politically connected owner, former fireman Wang Yizheng, have long enjoyed government favor in Dalian, so criticism of the company could also be seen as a protest against city leaders.

Dalian is also a power base for Bo Xilai, one of China’s expected future leaders and the former party boss of surrounding Liaoning province, and unrest in the city could be used to weaken his influence.

Concerns over pollution and chemical contamination are especially resonant in China, where growing incomes have created demands for a better quality of life and improved public services. Property values in Dalian, known for its popular beaches and clean air, may also have played a role.

The party hopes to tap into those aspirations to ensure its hold on power, forcing it to appear responsive in the wake of lifestyle concerns. And while similar protests have brought retaliation against the organizers, they’ve also produced results. Residents opposed to a paraxylene plant in the city of Xiamen in southeastern China marched in protest against it in 2007, persuading planners to move it instead to a less populated area of another southeastern city, Zhangzhou.

Paraxylene is widely used in the production of polyester for textiles and plastic bottles. Short-term exposure can cause eye, nose, or throat irritation in humans, and chronic exposure can affect the central nervous system and cause death.

While less polluting than many coal-burning power plants, the presence of such factories in China’s densely populated cities has become a hot-button issue for educated Chinese. ♦